With tightening environmental regulations, there is an increasing need for the measurement of particle emissions. In particular, the need for measurement is present in the development of filtering methods, in the research of various combustion processes, as well as in processes for monitoring actual emissions. In particle measurements, so-called cascade impactors have been conventionally used to classify the particles according to the particle size.
FIG. 1 shows a cross-sectional view of an impactor 10 consisting of several stages according to prior art. To simplify the figure, only the first two stages have been drawn. The flow 11 to be analyzed is sucked, for example by negative pressure, through the impactor 10. The air flow 11 is introduced through the frame structure 15 of the impactor to a first chamber 19a. Each stage comprises a nozzle part 12a; 12b equipped with orifices which are passed through by the flow carrying particles. Collection surfaces 13a; 13b are placed behind the nozzle parts 12a; 12b. The collection surface is provided with at least one outlet, through which the flow is allowed to flow to the next chamber or out of the impactor.
FIG. 2 shows a detail of the collection surface 13. The flow direction of the air flow 11 through the orifices of-the nozzle part is abruptly changed when it impacts upon the collection surface 13. Particles 21 carried by the flow 11 and having a sufficiently low mechanical mobility cannot follow the abrupt change in the direction of the flow but they hit the collection surface 13. The particles 21 having hit the collection surface 13 are deposited on the collection surface 13, forming a mass 22.
The mechanical mobility of the particles depends in a known way on the particle size. This makes it possible to classify the particles selectively according to the size. By selecting, in a known way, the number and size of orifices in the nozzle part 12a, 12b shown in FIG. 1, the distance between the nozzle part 12a; 12b and the collection surface 13a; 13b, as well as the flow rate to be used, it is possible to dimension each impactor stage in such a way that only particles having a mechanical mobility smaller than a desired value, i.e. being larger than a given particle size, are deposited on the collection surface 13a; 13b at each stage.
The successive stages can be dimensioned so that the first stage collects the largest particles (for example, particles with a diameter greater than 100 μm), the second stage collects the particles slightly smaller than these (for example, 10 to 100 μm), and the next stages would collect smaller and smaller particles, respectively. Thus, by measuring the masses 22 deposited on the collection surfaces 13 at the different collecting stages, it is possible to determine the size distribution of the particles in the flow under analysis. In conventional impactors, the mass deposited on the collection surface 13 is measured by weighing. In electrical impactors, an estimate of the mass deposited on the collection surface is made by monitoring the current caused by electric charges discharged by particles deposited on the collection surface.
A problem with the above-described impactor of prior art is the amount of work required for removing and reinstalling the collection surfaces. As mentioned earlier, in conventional impactors it is necessary to remove the collection surfaces for weighing; however, in electrical impactors it is also necessary to remove the collection surfaces, although not for obtaining the measurement result itself, as in conventional impactors. After a given measuring time, both impactor types require cleaning to remove the mass deposited on the collection surfaces. If this were not done, the collection surfaces would eventually be filled in such a way that the mass deposited on them would either impair the passage of the flow or, when disengaged, it would be carried to the next stage and cause an error in the measurements. To reduce this “blow off” effect, as well as the bouncing of particles hitting the collection surface, the collection surface 13 is typically treated with a substance which facilitates adhesion.
Both conventional and electrical impactors must be cleaned at regular intervals. Impactors of prior art are constructed in such a way that they must be disassembled part by part. For example, to disassemble the impactor of FIG. 1, first the shielding cover 15 would be removed, then the cover part 16 of the first stage and the supporting/shielding part 17a underneath the cover. Next, the nozzle part 12a of the first stage can be removed. After the removal of the nozzle part, it is possible to remove the collection surface 13a. After this, the supporting/shielding part 17b, the nozzle part 12b and the collection surface 13b of the second stage can be removed, respectively. In a corresponding manner, all the stages of the impactor are gone through. After the disassembly, the removed collection surfaces 13a, 13b can be weighed, if necessary. The partly disassembled impactor can then be cleaned, for example by placing the parts in a separate washing device. After the cleaning, the impactor is assembled in an order inverse to that presented above, after which the impactor can be used for measuring again.